Close all DB connections when Daemons sleep

Summary: Fixes T2933

Test Plan:
As guided by Evan - by setting $tasks = array(); in PhabricatorTaskmasterDaemon.php
and running 'phd debug taskmaster' and 'show full processlist' on mysql as root. No extra connections
detected.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2933

Differential Revision: https://secure.phabricator.com/D5654